There are many mechanical, electrical, and optical techniques for measuring the relative position of two articles. For example, a mechanical arm may be lengthened or shortened responsive to the movement of the articles. A change in an electrical property such as resistance or capacitance with relative position may be measured. Optical techniques such as light interference measurements or light attenuation in an attenuating medium are used when appropriate.
The available measurement techniques all have drawbacks in various applications. Mechanical measurement techniques impose loads on the system being measured, add substantial weight, are difficult to miniaturize, are difficult to provide with redundancy, and are subject to premature failures. Electrical measurement techniques are often limited to small changes in position, and are therefore not useful when the changes are on the order of many inches, feet, or more. They often require an exposed electrical contact, and usually at least a portion of the electrical measurement apparatus must move with the moving article so that the lead wires must also move. Electrical measurements also suffer from a high sensitivity to the environment of the sensor and to alignment errors. Optical position-measurement techniques typically require a line of sight between the articles, and are extremely sensitive to misalignment.
There is a need for an improved technique for measuring the relative position of two articles which overcomes these drawbacks. The present invention fulfills this need, and further provides related advantages.
The present invention provides a position sensor of the relative position between two objects that is based on the use of optical fiber technology. The position sensor is of low cost and high reliability, is highly sensitive to changes in position, and is easy to miniaturize. In some embodiments, there are no moving optical components, so no mechanical, electrical, or optical linkages to the moving article are required. The approach is insensitive to misalignment within normal tolerances. There is no additional mechanical loading to the moving article resulting from the position sensor, and no frictional forces that must be overcome.
In accordance with the invention, a position sensor comprises a light-source-and-light-emitting structure operable to emit light from each of an emitter plurality of light emitters disposed along an emitting length. The light-source-and-light-emitting structure typically comprises a light source such as a light-emitting diode. In a preferred form, the light-source-and-light-emitting structure comprises a light source, and a light-emitting optical fiber having an insertion end that receives a light input from the light source, and an emitter plurality of light emitters disposed along an emitting length of a lateral surface of the light-emitting optical fiber. The light emitters may be of any operable form, such as emitting notches in the light-emitting optical fiber or roughened emitting surfaces on the light-emitting optical fiber.
There is a first light detector having a first-light-detector light output, and a first light-collecting structure in a parallel-but-spaced-apart relation to the emitting length of the light-source-and-light-emitting structure. The first light-collecting structure has a first-light-collecting-structure extraction end that provides a first light output to the first light detector, and a first plurality of first-light-collecting-structure light collectors disposed along a first-light-collecting-structure collecting length of the first light-collecting structure in a facing relation to a first group of the respective light emitters of the light-source-and-light-emitting structure. There is, additionally, a second light detector having a second-light-detector light output, and a second light-collecting structure in a parallel-but-spaced-apart relation to the emitting length of the light-source-and-light-emitting structure. The second light-collecting structure has a second-light-collecting-structure extraction end that provides a second light output to the second light detector, and a second plurality of second-light-collecting-structure light collectors disposed along a second-light-collecting-structure collecting length of the second-light-collecting structure in a facing relation to a second group of the respective light emitters of the light-emitting structure. The first group and the second group of light emitters of the light-emitting structure are different. An opaque light shield is disposed between and movable parallel relative to the light-source-and-light-emitting structure and the light-collecting structures. A length of the opaque light shield measured parallel to the light-source-and-light-emitting structure is such that at least some of the first group and at least some of the second group of light collectors are not facing the light shield for at least some positions of the light shield.
There is desirably a sensor readout that receives the first-light-detector light output and the second-light-detector light output, and provides a responsive sensor output indicative of the position of the light shield. In one embodiment, the sensor output is responsive to a difference between the first-light-detector light output and the second-light-detector light output.
Preferably at least one of the first light-collecting structure and the second light-collecting structure comprises an optical fiber. The light collectors may be, for example, collecting notches in the light-collecting optical fibers or roughened collecting surfaces on the light-collecting optical fibers. There may be a second end remote from the extraction end of the light-collecting optical fibers, wherein the second end of the light-collecting optical fiber is internally reflective. In one preferred form, the first light-collecting optical fiber and the second light-collecting optical fiber are substantially coaxial.
The position sensor of the invention has a low manufacturing cost and is of high reliability. It may be readily scaled for emitting and collecting lengths, and may be made as large or as small as necessary. The only longitudinally extending components of the position sensor are the two optical fibers, which may each be less than 0.010 inch in diameter, and may be made smaller (or larger) if desired. These components may therefore be positioned in a small space on either side of the opaque shield. The two optical fibers may instead be larger diameter optical rods. The position sensor may be made with no moving optical components, by affixing the light shield to the moving element. Equivalently in an operating sense, the optical components may be affixed to the moving element. The present approach is relatively insensitive to maintaining a precise alignment between the optical fibers.
The position sensor is an optical device that achieves many of the same results as an electromechanical potentiometer that has a resistive element and a sensing element, but without its disadvantages. In the present approach, the sensing is achieved in a noncontacting manner by varying the amount of light transmitted to the light-collecting optical fiber rather than in a contacting manner as in an electro-mechanical potentiometer. The contacting elements of the electro-mechanical potentiometer can wear, resulting in degradation of performance as well as cross contamination of the contacting elements. The frictional forces of the contacting elements may also adversely affect its performance. The present approach has none of these disadvantages.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.